1. Field of the Invention
The present invention relates to a method for forming a planar microlens which is incorporated in a liquid crystal display device or the like, and a planar microlens which is obtained by this method.
2. Description of the Related Art
A molding method using photo-polymers is well known as a method for forming a planar microlens. In this molding method, a glass substrate, a stamper on the forming surface of which a number of nearly spherical concave portions or convex portions are formed and an uncured resin are used. The uncured resin is applied onto the glass substrate or the forming surface of the stamper. Next, the stamper is pushed onto the glass substrate. As a result of this, a press forming of the uncured resin is conducted, and thereby a lens layer made from a resin is formed on the glass substrate, Finally, the lens layer made from a resin is cured by applying an ultraviolet irradiation or the like.
This molding method is suitable for mass-producing a planar microlens at low cost. However, as a planar microlens which is incorporated in a communication device, an all-glass planar microlens is required to enhance qualities thereof.
As a method for mass-producing an all-glass planar microlens, methods using a reactive ion etching (RIE) have been known.
In one method using an RIE, a resin layer (photoresist layer) is formed on the surface of a glass substrate. An exposure is conducted upon the resin layer with a gray scale mask through which a certain amount of light is transmitted, and thereby the exposure amount is partially varied. Next, the resin layer is made to have a number of nearly spherical convex portions through a development process, The surface of the glass substrate is removed together with the resin layer by conducting an RIE to the resin layer, and thereby an all-glass planar microlens is obtained.
In another method using an RIE, a resin layer is formed on the surface of a glass substrate. An etching is conducted to the resin layer selectively to form insular portions thereof. Next, the resin portions remaining in an insular shape are heated and melted to whereupon they take on liquid-drop-shaped convex portions. An RIE is conducted to remove the convex portions, and thereby an all-glass planar microlens is obtained.
In the above-mentioned conventional methods, the lens portion formed on the surface of the glass substrate by an RIE has a thickness nearly equal to the thickness of the resin layer (photoresist layer) formed on the glass substrate in a case where the etching rate of the glass is equal to that of the resin.
In the conventional methods, in a case where the height of the convex portion of the lens or the depth of the concave portion is large (equal to or more than around 15 xcexcm), it is extremely difficult to uniformly form a resin layer (photoresist layer) having a thickness corresponding to the thickness of the lens as desired on the glass substrate. Further, since the convex portions formed through a radiant exposure through a gray scale mask or a melting of a resist remaining in an insular shape are limited to a predetermined shape are formed, the precise shape of the lens as desired cannot be accurately obtained.
To solve the above-mentioned problems, according to the first aspect of the present invention there is provided a method for forming a planar microlens comprising the steps of forming a stamper having a number of nearly spherical concave portions on the forming surface thereof, forming a resin layer having a number of nearly spherical convex portions on a glass substrate with said stamper, curing said resin layer, and thereafter completely removing said resin layer and forming minute convex lenses having the same shape as the nearly spherical convex portions of said resin layer on the surface of said glass substrate by a reactive ion etching (RIE).
According to the second aspect of the present invention, there is provided a method for forming a planar microlens comprising the steps of forming a stamper having a number of nearly spherical convex portions on the forming surface thereof, forming a resin layer having a number of nearly spherical concave portions on a glass substrate with said stamper, curing said resin layer, and thereafter completely removing said resin layer and forming minute concave lenses having the same shape as the nearly spherical concave portions of said resin layer on the surface of said glass substrate by a reactive ion etching (RIE).
The stamper in the first aspect of the present invention can be obtained by conducting a patterning to the surface of the glass substrate with the surface of the glass substrate being masked, and thereafter conducting a wet etching. It is also possible to form an intermediate mold by a Ni electroforming or a molding using photopolymers in which this stamper is used as a mother mold and to obtain a stamper by transferring this intermediate mold. An accurate transfer of the mold can thereby be achieved, and such a method has the advantage of mass-production.
As the stamper in the second aspect of the present invention, a Ni electroformed mold or a mold formed by a molding using photo-polymers can be used, which is formed by transferring the mother mold in the first aspect of the present invention one time.
Since the stamper and the resin layer which is formed with this stamper are comprised of a similar material, it is preferable to conduct in advance a release treatment to the forming surface of the stamper such as an application of a release agent or the like.
Further, in the present invention, when a material having a lower etching rate than that of the glass substrate is used as a material for the resin layer, it is possible to obtain a planar microlens comprising minute convex lenses having a greater radius of curvature in the peripheral portion of the lens than the top portion of the lens. As a result of this, it is possible to reduce a spherical aberration occurring in the peripheral portion of the lens.